The present invention is generally directed to an X-ray detector array for use in a computed tomography system, and more particularly to a method and apparatus for maintaining an X-ray detector array in a substantially isothermal and thermally stable condition.
A computed tomography (CT) imaging system typically includes an x-ray source and an x-ray detector array mounted on opposite sides of a gantry with an imaging area interposed between. The detector array typically includes a plurality of detector elements arranged in rows and columns. The detector array or module includes the detection elements and associated electrical components to convert the x-ray signal to either a measurable analog or quantifiable digital signal. In many configurations the array is mounted to the gantry on axially separated rails.
In operation the x-ray source generates x-rays that are directed at the array. When an object (e.g., the torso of a patient) is positioned within the imaging area, x-rays passing through the object are attenuated to different degrees, the varying degrees of attenuation dependent upon characteristics of the material through which the x-rays pass within the imaging area (e.g., bone may attenuate to a greater degree than flesh, etc.).
In CT, the gantry is used to rotate the x-ray source and detector array about an object to be imaged so that data corresponding to every angle is collected. Thereafter, the collected data is filtered, weighted and typically back projected by an image processor to generate one or more diagnostic quality images.
In image reconstruction, it is assumed that the gain of each detector remains constant throughout a data acquisition process and that any change in x-ray signal intensity at the detector is due to patient anatomy. Unfortunately, this assumption is not 100% accurate for several reasons. One particularly acute source of error in this regard has to do with how detector element operation is affected by element conditions during operation. More specifically, as is the case with many different electronic components, detector element response to a specific stimuli (e.g., a specific intensity x-ray) varies as a function of temperature.
There are several ways in which temperature affects element output and overall accuracy of acquired data. First, not surprisingly, temperature directly affects element output (or gain) during operation, the module can be subjected to temperatures different than the calibration temperature, resulting in uncorrected gain errors. Second, temperature gradients along array rails and between rails has been known to cause thermal distortions in the mechanical structure leading to uncorrected gain errors. In one particular use, when the relative temperatures of the center and ends of the array vary over time, image artifacts can be introduced. Third, other detector array components (e.g., photo diode associated with detector elements), are also affected by changes in temperature. Specifically the shunt resistance of a photo diode drops exponentially with temperature which results in leakage currents and generally a decrease in the signal to noise ratio.
When array output varies as a function of element and array environment temperature, the quality of resulting images is adversely affected. To this end, it has been observed that temperature effects on array output sometimes result in image artifacts that adversely affect the diagnostic usefulness of the resulting images.
There are many sources of heat in CT systems that directly affect the temperature of the array. Specifically, the X-ray tube used to generate the X-ray beam generates a large amount of heat in a CT system. In addition, motors, processors and other CT system components generate heat in the vicinity of the array. In recent years, the desire to increase patient throughput (i.e., the number of acquisition sessions performed per day) has fueled the use of more powerful x-ray sources so that the amount of data required to generate images can be acquired in a shorter period of time. These higher powered systems, while appreciably faster than their predecessors, have only exacerbated the array heating problem and the associated image degradation.
To address temperature related array operation problems, the industry has developed various solutions aimed at maintaining nearly isothermal and thermal stable arrays. To this end, accepting that elements will heat during operation, most solutions provide some type of element heating configuration that is mounted with the array on the rails. The heating configuration is generally used to heat the elements approximately to an expected high temperature level and to maintain that temperature level throughout an acquisition period. The heater control point is set to be consistent with the expected high temperature limit and the maximum allowable module temperature change.
Unfortunately, in high power systems the array temperature can exceed the temperature at which acceptable image quality is achieved. Under these circumstances, a heating control cannot maintain the detector array within an acceptable operational temperature.
There remains a need, therefore, for a simple and economic method for maintaining a detector array at a constant temperature, and particularly for maintaining a detector array at a constant temperature when operated in conjunction with high-powered X-ray tubes wherein the highest temperature is greater than what is permitted by the imaging system.